In general, Southerners are somewhat untrusting of outsiders. Alabama during the time of To Kill a Mockingbird is no exception. The Civil War was still current in the minds of some Maycomb citizens, and there are quite a few references to it throughout the novel. Miss Caroline and her new-fangled teaching style is much like the Yankee carpetbaggers who invaded the South after the war, bent on making money and taking advantage of its down-trodden citizens. Harper Lee's creation of a young outsider makes her seem suspicious to both the children and their parents alike. The fact that she is from Winston County in Northern Alabama--and apparently very proud of it--makes her even more questionable. Lee reveals that Winston County itself seceded from Alabama after the state's secession from the U.S. prior to the Civil War, making citizens of Winston County the equivalent of "Yankees." Simply put,
North Alabama was full of Liquor Interests, Big Mules, steel companies, Republicans, professors and other persons of no background.
No doubt these are the young Scout's words (and not in adult retrospect), remembered and repeated from the gossip of Miss Stephanie or other neighbors (and possibly even some from Atticus himself). Not only is Miss Caroline an outsider and a newcomer to Maycomb, her condescending attitude marks her as a modern day scalawag--a Southerner who supported the Reconstruction policies of the Union--and not a person that will fit into Maycomb's little world.
Lee accomplishes the alienation of Miss Caroline through geography. The fact that she is from a place outside of Maycomb, and particularly a more "metropolitan" part of Alabama, distinguishes and differentiates her from the native folk of Maycomb County.
In the minds of Maycomb's citizenry, outsiders (much like blacks) are "not to be trusted," in the words of Atticus's closing argument. Anyone or any influence from beyond the lines of their myopic world is unwelcome, and as such, Miss Caroline becomes the object of gossip and ridicule among the townspeople for her background.